Typical roll-up doors comprise a flexible curtain that when the door is open the curtain is wound about a roller above the doorway. To close the door, the curtain unwinds as two vertical tracks guide the curtain down across the doorway. Roll-up doors are typically either powered open and closed or are powered open and descend controllably by gravity to close.
Some roll-up doors are powered by a drive unit that can rotate the curtain's roller in either direction to open or close the door. Other roll-up doors are powered by a drive unit that drivingly engages the curtain itself while the roller takes up any curtain slack as the door opens.
Many roll-up doors in use today, especially larger ones, employ windbars to assist in reducing the negative effects of wind loading. Large doors present a substantial cross-sectional area for the wind to react with. Negative effects include pulling the door curtain edges out of the guide tracks, excessive bowing of the curtain, and increasing the frictional forces between the tracks and the curtain edge to a point where the door will not open and/or close. Some windbars are integral to the door curtain itself. Other windbars are separate and travel relative to the curtain as the door opens and closes.